The present invention relates to an apparatus that controls pivoting of an axle in an industrial vehicle such as a forklift. More particularly, the present invention pertains to an axle pivot control apparatus for locking an axle, which is pivotally supported by a vehicle's body frame
A typical industrial vehicle such as a forklift includes a rear axle that pivots relative to the body frame. The rear axle pivots such that all the wheels always contact the ground even if the vehicle runs over bumps and dips on the road surface. This structure improves comfort, handling and stability of the vehicle.
Japanese Unexamined Patent Publication No. 58-211903 describes a forklift that has a locking mechanism for locking the rear axle against pivoting when lateral acceleration acting on the forklift exceeds a predetermined threshold value. Even if a great lateral force acts on the forklift, tilting of the forklift body frame is minimized. The forklift thus turns in a stable manner.
Japanese Unexamined Patent Publication No. 58-167215 describes a forklift that has a locking mechanism that locks the rear axle against pivoting when the weight of a load on forks and the height of the forks exceed predetermined threshold values. When the load is heavy and the forks are high, the center of gravity of the forklift is raised, which destabilizes the forklift. However, the mechanism of the publication No. 58-167215 locks the rear wheels in such cases. The forklift therefore turns or loads and unloads in a stable manner.
The mechanism according to the publication No. 58-167215 determines whether to lock the rear axle in accordance with the weight of the load on the forks and the height of the forks regardless of the magnitude of the lateral acceleration acting on the body frame. However, even if the forks remain at a certain height, changes in the lateral acceleration change the side-to-side stability of the vehicle. Therefore, the mechanism of the publication No. 58-167215 requires that the threshold value regarding the fork height be relatively small so that the side-to-side stability of the vehicle is unaffected by a great lateral acceleration. When the fork height is greater than the threshold value, the rear axle is locked even if there is little or no lateral acceleration acting on the body frame, that is, even if the vehicle would be stable without locking the rear axle.
If the functions of the mechanisms of the publications No. 58-211903 and No. 58-167215 are combined, a threshold value regarding the lateral acceleration is employed as well as the threshold values regarding the load weight and the fork height. The rear axle is locked when the lateral acceleration is greater than the threshold value and when the threshold values of the load weight and the fork height are exceeded. Such a combination allows the threshold value of the fork height to be maximized. Therefore, the rear axle is not unnecessarily locked.
However, a greater threshold value regarding the fork height requires that the threshold value of the lateral acceleration be smaller such that the vehicle is stable when turning with the forks held relatively high. When the lateral acceleration is greater than the threshold value, the rear axle is locked even if the forks are relatively low, that is, even if the vehicle would be stable without locking the rear axle. Therefore, the threshold value of the fork height cannot be increased without considering the threshold value of the lateral acceleration. As a result, combining the functions of the two publications does not eliminate unwanted locking of the rear axle.
Unnecessary locking of the rear axle is contrary to the goal of improving the comfort, stability and handling of the vehicle by allowing the rear axle to pivot. For example, the center of gravity of the forklift is relatively close to the rear wheels when there is no load on the forks. In this state, if the rear axle is locked based on the lateral acceleration, running over bumps and dips on the road surface raises one of the front wheels. If the front wheels are driving wheels, decreased ground contact pressure causes the wheel to spin. The center of gravity is relatively close to the front wheels when the forklift carries a relatively heavy load. If the rear axle is locked in this state and one of the rear wheels runs over a bump, the other rear wheel loses contact with the road surface. Since the rear wheels are steered wheels, this reduces the steering effectiveness.
If the rear axle is locked, the rear portion of the forklift is raised by a greater amount when one of the rear wheels runs over a bump compared to the case where the rear axle is not locked. Locking of the rear axle therefore may destabilize the vehicle. Further, locking the rear axle makes the body frame react to the road surface, which reduces the riding comfort of the forklift. It is therefore imperative to reduce unnecessary locking of the axle.